Data compression occurs in a number of contexts. It is very commonly used in communications and computer networking to store, transmit, and reproduce information efficiently. It finds particular application in the encoding of images, audio and video. Video presents a significant challenge to data compression because of the large amount of data required for each video frame and the speed with which encoding and decoding often needs to occur. The advanced Audio and Video coding Standards (“AVS”) is an existing audio and video coding standard developed in China. Work is set to begin on developing a next-generation standard to replace AVS, which may be known as AVS2.
Many video coding standards use block-based coding processes. In these processes, the image or frame is divided into blocks, typically 4×4 or 8×8, although non-square blocks may be used in some cases, and the blocks are spectrally transformed into coefficients, quantized, and entropy encoded. In many cases, the data being transformed is not the actual pixel data, but is residual data following a prediction operation. Predictions can be intra-frame, i.e. block-to-block within the frame/image, or inter-frame, i.e. between frames (also called motion prediction).
To compress data, the coding of the quantized transform coefficients is designed to take advantage of characteristics of the transform to improve compression. In the case of AVS, the coefficients are coded using a quadruple: level, run, sign, end-of-block (EOB) flag. The coding is in a reverse zig-zag direction starting from the last non-zero coefficient in a transform block (hence the requirement for the EOB flag).
The level-minus-one and run data is binarized using unary binarization and the bins are then coded using context-based entropy coding. AVS specifically uses arithmetic coding for the transform coefficient data.
There are some drawbacks to the existing coding scheme for AVS. In some cases, long runs can occur. In the case of a 32×32 block, the worst case run is 1023. When binarized, this results in a number with 1024 bins to be coded. The EOB flags present a similar worst-case overhead. For example, with a 32×32 block the worst case scenario is 1024 non-zero coefficients, which means 1023 EOB flags. Finally, due to the coding scheme adopted for AVS, the decoder does not have any a priori knowledge regarding the location of transform coefficients within the block until all transform coefficients have been decoded. This prevents this data from being used to improve context determination or other coding decisions.
Similar reference numerals may have been used in different figures to denote similar components.